Add fixtures / unit testing for the stock item testing framework

InvenTree/part/fixtures/test_templates.yaml

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+# Tests for the top-level "chair" part
+- model: part.parttesttemplate
+  fields:
+    part: 10000
+    test_name: Test strength of chair
+
+- model: part.parttesttemplate
+  fields:
+    part: 10000
+    test_name: Apply paint
+
+- model: part.parttesttemplate
+  fields:
+    part: 10000
+    test_name: Sew cushion
+
+- model: part.parttesttemplate
+  fields:
+    part: 10000
+    test_name: Attach legs
+
+- model: part.parttesttemplate
+  fields:
+    part: 10000
+    test_name: Record weight
+    required: false
+
+# Add some tests for one of the variants
+- model: part.parttesttemplate
+  fields:
+    part: 10003
+    test_name: Check that chair is green
+    required: true
+
+- model: part.parttesttemplate
+  fields:
+    part: 10004
+    test_name: Check that chair is especially green
+    required: False